The size of a 1/2 index card can vary depending on the standard index card size being used. In the United States, the most common standard index card size is 3 inches by 5 inches (3x5). A 1/2 index card would be half the size of this standard, resulting in a size of approximately 1.5 inches by 2.5 inches (1.5x2.5).
In some cases, a 1/2 index card may be referred to as a "half-size" index card. For example, Oxford offers half-size index cards that are 3 inches by 2.5 inches (3x2.5). Mead also provides half-size color index cards with the same dimensions.
